The gene encoding the Green Fluorescent Protein (GFP) of the jellyfish Aequorea Victoria has been expressed in heterologous systems and proteins with spectral characteristics identical to those of wild-type protein produced. This has led to the widespread use of GFP as a marker for gene expression and as a protein tag, both in cell culture and in multicellular organisms. Using combinatorial mutagenesis and digital imaging spectroscopy we aim to produce and select mutants of GFP which have altered excitation and/or emission spectra. A set of proteins will be created, each of which produces a different fluorescence color. This will enable the simultaneous analysis of more than one promoter or protein per cell or organism. PROPOSED COMMERCIAL APPLICATION: The green fluorescent protein and its colored derivatives can be used as protein tags in gene expression without the addition of exogenous substrates. In consideration of the multitude of applications which are developing for GFP alone, we envisage that such proteins will have broad commercial applications including: organismal development, drug-screening and diagnostic assays.